The survival of pathogens in the human body has been rigorously studied for well over a century. Bacteria are able to colonize, persist and thrive in vivo due to an array of capabilities. Most bacterial pathogenesis studies have focused on mono-culture infections; however, it is clear that many bacterial infections are not simply the result of colonization with a single species, but are instead a result of colonization with several. Microbes within polymicrobial infections often display synergistic interactions that result in enhanced colonization and persistence in the infection site, and the molecular processes controlling these synergistic interactions are not well defined. Our lab utilizes a two-species model system to study polymicrobial synergy. The system is composed of the opportunistic Gram-negative pathogen Aggregatibacter actinomycetemcomitans (Aa) and the Gram-positive bacterium Streptococcus gordonii (Sg). Using this model system, we are testing the overriding hypothesis that bacteria within polymicrobial infections display defined responses to the primary metabolites produced by other members of the microbial community, and these responses are critical for establishing polymicrobial infections. We have primarily focused on elucidating the molecular responses of Aa to two primary metabolites produced by Sg, L-lactate and H2O2. These studies have uncovered novel Aa responses that not only affect how this bacterium interacts with Sg but also how it interacts with the host. The overall goals of this research plan are to 1) examine from a mechanistic standpoint, how polymicrobial interactions between oral bacteria impact community development, resistance to host innate immunity, and in vivo persistence, and 2) develop novel technologies for probing polymicrobial interactions. To this end, we have proposed experiments to (i) elucidate the molecular mechanism of Aa L-lactate preference and assess its importance in vivo, (ii) elucidate the mechanism of Aa protection from innate immunity during co-culture and assess its importance in vivo, (iii) characterize the impact of H2O2 and co-culture on biofilm dispersion.